Alongside magnetic resonance tomography (MR), positron emission tomography (PET) has also become increasingly widespread of recent years in medical diagnosis. While MR tomography is an imaging method for displaying structures and slice images in the interior of the body, PET enables a visualization and quantification of metabolic activities in vivo.
PET uses the particular properties of positron emitters and positron annihilation in order to determine the function of organs or cell areas quantitatively. In this case, before the examination the patient is administered appropriate radiopharmaceuticals that are marked with radionuclides. In the event of decay, the radionuclides emit positrons that interact with an electron after a short distance, resulting in a so-called annihilation. Two gamma quanta are produced in this case and fly apart from one another in opposite directions (offset by 180°). The gamma quanta are detected by two opposite PET detector modules inside a specific time window (coincidence measurement), as a result of which the location of the annihilation is determined at a position on the connecting line between these two detector modules.
For detection, in the case of PET the detector module must generally cover a major part of the length of the gantry arc. Said module is subdivided into detector elements with a side length of a few millimeters. When detecting a gamma quantum, each detector element generates an event record that specifies the time and the detection location, that is to say the appropriate detector element. These items of information are transferred to a fast logic unit and compared. If two events coincide with a maximum time spacing, it is assumed that there is a gamma decay process on the connecting line between the two associated detector elements. The reconstruction of the PET image is performed with the aid of a tomography algorithm, that is to say the so-called back projection.
A superposed imaging of the two methods is desirable in many instances on the basis of the different items of information that are obtained by MRI and PET.
In the field of tomographic measuring methods the problem arises namely that a positron emission tomography system (PET) can only deliver functional images and not anatomical ones. Combining the imaging methods of MRI and PET into one unit and making it possible to use them simultaneously, as far as possible, is a development goal for future systems. In order to be able to undertake an assignment of the recorded functional areas to the anatomical structures, a magnetic resonance tomography unit is combined with a PET unit, generally by fitting a PET measuring device into an MR unit. However, in doing so, the problem arises that the PET measuring device must not interfere with the MR measurements and vice versa—something that can happen due to the physical principles used.
Until now it has been usual to combine the two measuring devices by design appropriate shielding of the whole system and by providing measures between the two devices. One design measure with a specific design can be, for instance, to avoid the magnetic components in the PET measuring device. One appropriate measure against the incoming and outgoing radio-frequency radiation would be an electromagnetic radiation-tight screening cover; however, the realization thereof is often difficult and/or complex, so that, in general, this is refrained from.